Cisco IOS (Internetwork Operating System) is the operating system that is used on some of Cisco’s routers and switches. It’s a multitasking operating system with a monolithic architecture. This means that the operating system runs as a single image where all processes share the same memory. When a single process crashes, it could affect all other processes. Cisco also has operating systems like IOS-XR where processes are separated. Cisco IOS uses the CLI (Command Line Interface) for most of its configuration, users have to use a variety of commands to configure the router or switch.